The present invention relates to a sintered stainless steel exhibiting markedly improved resistance to stress corrosion cracking and a process for the production thereof, the steel comprising a matrix phase of an austenitic or ferritic-austenitic duplex structure and a dispersed phase of an austenitic structure.
In particular, the present invention relates to a sintered stainless steel which exhibits markedly improved resistance to stress corrosion cracking under a CO.sub.2 -H.sub.2 S-chloride ion-containing environment (referred to hereunder as "Co.sub.2 -H.sub.2 S-Cl.sup.- environment").
In recent years, gas and oil wells have been exploited deep under ground under increasingly severe conditions. The depths of gas and oil wells sometimes reach 10,000 meters below the ground. Seamless steel tubes used in assemblying oil and gas wells, i.e., drilling pipes, and tubing and casing pipes (hereinafter collectively referred to as "oil well tubing and casing pipes") are used under much more severe corrosive and mechanical conditions. Newly developed oil and gas wells are in general characterized in that the oil is sweet and sour, and the temperature and pressure are also increasing. Namely, the oil well tubing and casing pipes are used in an environment containing a large amount of CO.sub.2 gas, H.sub.2 S gas, and concentrated Cl.sup.- ions at a high temperature and pressure.
Under those severe conditions, in place of a conventional low Cr steel, a martensitic 13Cr steel has widely been used. However, even such 13Cr steel is not free from general corrosion and pitting corrosion under the severe conditions in a deep well. In case of a strengthened steel, sulfide stress corrosion cracking easily occurs in the presence of a minor amount of H.sub.2 S.
Thus, in oil and gas wells in which the concentration of H.sub.2 S gas is relatively high, even 13Cr steel does not exhibit satisfactory resistance to corrosion. Therefore, in the past few years in place of 13Cr steel, a ferrite-austenite duplex stainless steel which contains 22-25% of Cr has been used.
Duplex stainless steel is a stainless steel which contains two types of phases and exhibits a threshold stress higher than that of austenitic or ferritic stainless steel which contains Cr at the same level. Furthermore, duplex stainless steel is satisfactory in respect to its resistance to SCC, tensile strength, and toughness.
As is well known in the art, duplex stainless steel is characterized by a high threshold stress value against SCC. FIG. 1 shows graphs disclosed in the Journal of Corrosion Engineering, vol. 30, No. 4, pp. 218-226 (1981) by one of the inventors of the present invention.
FIG. 1 shows graphs of the SCC resistance determined for 25Cr-6Ni duplex stainless steel (designated by the symbol "O"), 28Cr-4Ni ferritic stainless steel, the composition of which corresponds to that of the ferritic phase of the duplex steel (designated by the symbol ".cndot."), and 21Cr-9Ni austenitic stainless steel, the composition of which corresponds to that of the austenitic phase of the duplex steel (designated by the symbol ".DELTA."). After preparing these three types of steel through an ingot making process, corrosion tests were carried out using a 427K, 45% MgCl.sub.2 solution. Graph (a) shows the relationship between the applied stress and the time to failure. Graph (b) shows the stress ratio, i.e. the ratio of the threshold stress against SCC to the 0.2% yielding point (.sigma..sub.th /.sigma..sub.0.2) plotted with respect to the time to failure. The higher the ratio, the better is the SCC resistance.
As is apparent therefrom, 25Cr-6Ni duplex stainless steel (designated by "O") exhibits a .sigma..sub.th /.sigma..sub.0.2 ratio higher than those of 21Cr-9Ni steel (designated by ".DELTA.") and 28Cr-4Ni steel (designated by ".cndot.") at a time to failure of 600 hours or longer. This means that the resistance to SCC of the duplex stainless steel is much better than that of the austenitic or ferritic stainless steel.
Fontana et al. first reported concerning why duplex stainless steel can exhibit such improved resistance to SCC as described above and said that due to its chemical composition the ferrite phase causes a keying effect by which duplex stainless steel can exhibit such improved properties. Uhlig et al and Shimodaira et al also reported their investigations on the mechanism of such a keying effect.
Furthermore, one of the inventors of the present invention disclosed in the previously mentioned paper that the resistance to SCC of duplex steel does not depend on the chemical composition of each of the constituent phases, i.e., matrix phase and dispersed phase, but on the structure in which the two phases are present in a mixed state. That is, the resistance to SCC is derived from a keying effect caused by the presence of an austenitic phase dispersed in a discrete state in a matrix phase.
FIG. 2 schematically illustrates the above-described mechanism of SCC propagation in a conventional duplex stainless steel, which was prepared using an ingot making process. In this figure, the thick line indicates the path along which the SCC propagates. It is apparent that the SCC resistance of a duplex stainless steel is first determined by that of the ferritic phase contained therein. Therefore, if the ferritic phase exhibit improved SCC resistance, the duplex steel can exhibit improved SCC resistance. If not, as shown in FIG. 2, the SCC propagates through a ferritic phase, detours a discrete phase, and stops upon reaching another austenitic phase in conventional duplex stainless steels.
FIG. 3 is a graph which one of the inventors of the present invention disclosed in "Journal of Materials for Energy Systems", Vol. 5, No.1, June 1983, pp. 59-66. The graph summarizes test results of the SCC resistance of a ferrite-austenite duplex stainless steel in a CO.sub.2 -H.sub.2 S-Cl.sup.- environment. Typical, commercially available duplex stainless steel includes 22Cr series (22Cr-5.5Ni-3Mo) and 25Cr series (25Cr-7Ni-3Mo) steels. The graph shown in FIG. 3 was obtained by carrying out a corrosion test using 25Cr series steels in a 25% NaCl solution containing CO.sub.2 at 30 atms with varying temperature and P.sub.H.sbsb.2.sub.S. In this figure, the symbols ".cndot." and " 38 indicate cases in which SCC occurred. As is apparent from FIG. 3, for practical purposes the upper limit of P.sub.H.sbsb.2.sub.S is 0.1 atm. It was observed that a preferential attack took place on the ferritic phase and the SCC originated from the area where the preferential attack occurred.
Needless to say, in order to further improve the resistance to corrosion it is easily anticipated by those skilled in the art to increase the Cr content of the duplex steel. However, the higher the Cr content, the more easily .sigma. (sigma) phase forms, making the working thereof practically impossible.
Thus, at present, an application in which the use of the duplex stainless steel might cause troubles in respect to the resistance to corrosion has required the employment of Hastelloy C276 (tradename: 15Cr-16Mo-3.4W-1.0Co-60Ni-Bal. Fe), MP 35N (tradename: 20Cr-10Mo-35Co-35Ni-Bal. Fe), etc. However, these alloys are quite expensive, since they contain a relatively large amount of expensive alloying elements such as Mo, Co, and Ni. In addition, their hot workability and productivity are not satisfactory, since they contain a relatively large amount of these alloying elements.